1. The Field of the Invention
This invention relates to heat pipes, and more particularly to two-dimensional flows of liquid and vapor through wickless channels to maximize flows of vapor and liquid, minimize the effect of obstructions and hot spots, and minimize temperature differentials between a source of heat and a heat sink.
2. The Background Art
Heat exchangers are well documented throughout engineering literature as mechanisms for exchanging heat between media, moving heat from one place to another, and other means for energy transport. Typically, heat is transferred by conduction, convection, radiation, or a combination thereof. Heat transfer may or may not include a phase change of a working fluid involved in the heat transfer. That is, for example, a material may operate as a working fluid and change phase between liquid and vapor in order to capitalize on the large latent heat value of vaporization. Similarly, certain heat transfer mechanisms used in space craft and for highly sensitive optical detection may rely on the latent heat of vaporization or the latent heat of phase change from solid to liquid or vapor found in materials such as helium, hydrogen, nitrogen, water, alcohol, refrigerants or the like.
One mechanism for heat transfer has been embodied in a heat pipe. Heat pipes generally rely on a hot end that receives thermal energy from a source of heat. At the hot end of the heat pipe, a temperature differential between a heat source and a working fluid inside a sealed heat pipe will drive heat into the working fluid, vaporizing the liquid. The vapor, due to its expansion and resulting pressure differential will flow to the opposite or cold end of the heat pipe. At the cold end of the heat pipe, a heat sink in thermal contact with the pipe receives heat from the vapor, sealed within the heat pipe, but releasing energy to the pipe, which then transfers it to the heat sink. Upon loss of the thermal energy at the cold end of the heat pipe, the working fluid in its vapor phase condenses to a liquid phase.
Liquid accumulating at the cold end of a heat pipe condenses into a wick member, by which is meant generally and herein as a bundle of fibers of such comparatively small size as to promote capillary action there along due to surface tension as a dominant force acting on liquids therein. The wick carries the liquid by capillary action from the cold end of the heat pipe back to the hot end of the heat pipe. Liquid arriving at the hot end of the heat pipe then repeats the cycle of vaporization, transport, condensation, and return by capillary action from the cold end to the hot end.
In many environments, relatively modest temperature differentials are available. That is, for example, in a furnace, hundreds if not thousands of degrees of temperature difference may exist between one portion of the device and another. By contrast, in space craft, optical devices, electronic equipment, and the like, it may be highly desirable for the equipment to operate very close to the temperature of a sink to which heat is rejected, or even ambient temperatures. Operating close to ambient temperatures provides very little temperature differential to drive heat transfer. Typically, heat pipes require temperature differentials of many degrees. For example, it is not uncommon to find a heat pipe in which the temperature differential between a heat source and the liquid in the wick is over 20° due to the thermal impedance between the heat source or sink external to the pipe and the surfaces inside the pipe where vaporization and condensation occur. Larger temperature differentials drive larger amounts of heat. Nevertheless, developing a heat pipe mechanism that can operate with smaller temperature differentials would be very desirable.
It has been determined that most of the temperature drop between a heat source, or even between the outer wall of a heat pipe, and the liquid at the hot end of a heat pipe actually occurs across a vapor gap. That is, liquids have a property called surface tension, This is a measure of the tendency of a liquid to adhere to itself. Liquids may also have a certain surface tension characteristic with respect to solids within which they may come in contact. That is, a liquid has an adhesive property with respect to a solid, and sometimes a repulsion property, since some liquids and some solids actually do not adhere.
Nevertheless, a liquid tends to adhere to itself during flow according to its mass, viscosity, flow conditions, and its surface tension. At small relative dimensions, surface tension becomes a very significant property and the surface tension forces become significant in the overall flow of a working fluid.
Thus, in a heat pipe, liquids tend to adhere to themselves. The result is that the liquid adheres to itself in a body or stream within a wick, leaving a vapor-gap between the liquid in the wick, and the solid wall against which a wick may rest. Thus, although a wick theoretically maintains a liquid film within itself, and against a heated wall, the practical reality is otherwise.
This is perhaps not amazing, since the hot end of a heat pipe is continually generating vapor. Thus, the fluid dynamics and heat transfer characteristics in the hot end of a heat pipe would be expected to maintain both liquid and vapor phases in close proximity. Accordingly, the liquid phases tend to stay to themselves within the wick material, while the vapor phases tend to be generated at the hottest portions and migrate away therefrom toward the cooler end of the heat pipe.
Thus, vapor gaps cause very large relative temperature differentials to exist between the hot wall of a heat pipe, and the liquid surface of a working fluid in the wick. Depending on the heat flux, and the particular dynamics and dimensions of the heat transfer system, a heat pipe using water or alcohol at or near ambient conditions may have a substantial temperature differential between the hot wall and the liquid interface in a wick carrying liquid working fluid back to the hot end of the heat pipe.
Meanwhile, at the opposite or relatively cooler end of a heat pipe, both phases of the working fluid exist again. That is, the vapors must contact a cool surface at the cold end or cool end of the heat pipe, condense thereat, then make their way into the wick to begin their passage back to the hot end. The accumulation and collection process is necessarily occurring on a very small scale. The scale can actually be at a molecular level, as the individual molecules of liquid condense and agglomerate.
Once again, liquid will tend to adhere to itself, and a vapor layer may exist between a liquid surface in a wick, and the cold end of the heat pipe. That is, at the wall, liquids may actually not come in contact with the surface of the heat pipe, on the inside. For example, vapors may simply contact liquids in the wick and condense. The liquid surface tension may tend to keep liquid away from the wall. As a practical matter, with such dynamics of fluid and heat flows, a large temperature differential may also exist at the cool end of a heat pipe. The temperature differential is of course commensurate with the temperature differential at the hot end. Thus, it appears that a vapor jacket or layer may still form due to the inherent property of surface tension.
As a practical matter, a thinner capillary wick completely full of liquid might tend to avoid establishment of a vapor layer between a wall and a liquid surface in a wick. However, wicks cannot be so easily controlled, and the dynamic activity of a heat exchange system does not necessarily lend itself to an optimization of a wick thickness. That is, the amount of liquid, the flow of liquid, the temperature differentials, the flow pressures, and the like are dynamics along the entire length of a heat pipe. Accordingly, optimizing for any one location would typically move another process a different location out of its optimal set of parameters.
All systems have bottlenecks. That is, in any process, including chemical reactions, industrial processes, traffic patterns, heat pipes, mass flows, heat transfer paths, and the like some limiting rate occurs at some limiting location. Very few systems are optimized to operate at their perfect conditions at all locations within the system, and particularly not at all times and under all conditions. Such is antithetical to the principles of physics. Thus, vapor flows may be obstructed or restricted due to the fluid dynamics of their passage along some conduit or enclosed path. Likewise, the flow of liquids may be restricted by the fluid dynamics of the liquid in its environment, as well as various obstructions along the path including turns, blockages, and the like.
Thus, it would be an improvement in the art of heat pipes to provide two situations that do not appear present in heat pipe design. The first is to provide a mechanism to avoid obstructions, and thus permit vapors to flow in any direction between a comparatively hotter area of a heat pipe, and a comparatively cooler area of a heat pipe. Similarly, it would be an advance in the art to provide a multiplicity of paths for liquids, or a liquid working fluid within a heat pipe, to travel from a condensate location near the cooler portion of the heat pipe back to a hotter area where it is needed for flooding and cooling an area. Likewise, if a heat pipe has a particularly, comparatively hotter portion that tends to dry out, it would be an advance in the art to be able to provide liquid from multiple directions to that location in order to reflood the area as vapor is boiled off.
Likewise, it would be an advance in the art to develop a mechanism whereby separation of a liquid surface from the wall of the heat pipe is minimized. It would be a significant advance in the art to implement a mechanism whereby the liquid is kept in contact with the wall of a heat pipe, preferentially, thus allowing no vapor layer to interfere. Thus, the temperature differential between an outer wall, and a liquid surface against an inner wall of a heat pipe would be substantially minimized.
It would be another advance in the art if the thickness of a wall of a heat pipe were minimized in order to reduce conductive resistance. Thus, it would be an overall improvement in the art of heat pipes to create a heat pipe having little or no wick, yet providing good adhesion of a liquid working fluid against a wall near the hotter end of a heat pipe, in order to maintain flooding of the area with liquid heat transfer fluid.
Likewise, it would be a substantial advance in the art to provide flooding of a hot area of a heat pipe from multiple directions with equal ease, effectively isotropically with respect to all available flow directions, providing a substantially uninhibitable flow, from multiple directions in or out of plane with respect to a hot spot. It would be another advance in the art if liquids could be kept at or near the surface, flooding the hot areas of a heat pipe, by returning from the cooler areas of the heat pipe through multiple available paths.
It would also be an advance in the art to minimize the stripping (e.g., entrainment) of liquid out of the liquid flow by passing vapors.
In heat pipe design vapors are typically provided larger volumes in which to travel than are liquids, vapors typically have a volume on the order of one thousand times greater that the equivalent mass of a liquid. Accordingly, vapors traveling through the same space as liquids must have one thousand times the volume (or whatever their vapor-to-liquid volume ratio is and a corresponding ratio of velocities on a given area.) However, at the interface between a liquid traveling from the relatively hotter end to the relatively cooler end of a heat pipe, vapors travel at a relatively high rate of speed, stripping liquid from the surface of the liquid traveling in the other direction. Entrainment of liquids into vapor flows tends to inhibit reflooding the hot end of the heat pipe by liquids. Thus, a mechanism for separating the flows, may be desirable. A mechanism for automatically separating the flows, and sharing the available volume while still stabilizing the liquid flows against entrainment by vapors would be an advance in the art.